Colloquium Spring 2005
Martin Luther King, Jr. Day, University Holiday
Not all noise in experimental measurements is unwelcome. Certain types of fundamental noise contain valuable information about the system itself -- a notable example being the inherent voltage fluctuations that exist across the terminals of any resistor (Johnson noise), from which the electron temperature may be determined. In magnetic systems, fundamental noise can exist in the form of random spin fluctuations. Felix Bloch noted in 1946 that statistical fluctuations of N paramagnetic spins should give rise to measurable noise of order sqrt N spins, even in zero magnetic field. I will address precisely these same sqrt N spin fluctuations, and show how one can get the valuable spectroscopic information by listening carefully to the noise. I would argue that noise spectroscopy is a valuable technique that is particularly suitable for nanoscale systems where fluctuations are large. Related to noise is dissipation that is responsible for inelastic electron tunneling spectroscopy (IETS). I will consider applications of the closely related inelastic tunneling spectroscopy to macromolecules and DNA. References: Spectroscopy of spontaneous spin noise as a probe of magnetic resonance and spin dynamics , Nature, v 431, p 49, (2004); Novel spin Dynamics in a Josephson Junction, PRL, v 92, p 107001, (2004).
Confined water and adsorbed hydrogen in carbon nanotubes: A neutron-scattering study and its relevancy to bionanotechnology
Intense Pulsed Neutron Source, Argonne National Laboratory
- Joint colloquium with Astronautics and Space Technology Division
Solar System Frontier: Where the Solar Wind Meets Interstellar Medium
Astronautics and Space Technology Division, Viterbi School of Engineering, University of Southern California
Coronas and glories: phenomenon, explanations and experiments
University of Applied Sciences , Brandenburg, Germany
Coronas and glories are very spectacular atmospheric optical phenomena. However, their theoretical explanation is not as simple as usually anticipated. The talk will focus on the natural phenomenon, it then give a quick tour on explanations ranging from simple Fraunhofer diffraction theory with its limitations to the complete electrodynamic Mie theory and it finally describes experiments to study the phenomenon in the laboratory.
Presidents Day, University Holiday
The history and current state of water on Mars
Jet Propulsion Laboratory
No Colloquium: March APS Meeting
Computational tools are playing an increasingly important role in understanding and controlling matter at the microscopic scale, by predicting with quantitative accuracy the property of materials based on their atomic and molecular constituents. In the next decade, the coming of age of first principles theories of matter and related computational techniques --as well as the growth of computer power-- will allow one to simulate a wide variety of alternative materials with desired properties, and thus the engineering of optimized materials from first principles will become possible. In this talk, computer simulations using First Principles Molecular Dynamics (FPMD) and Quantum Monte Carlo (QMC) techniques are employed to solve specific nanoscience problems, in particular to investigate the physical properties of group IV semiconductor nanostructures and their possible use as chemical and biological labels. While robust experimental results have been established for II-VI nanocrystals in the last decade, group IV elemental nanostructures are much less well characterized. The interplay between quantum confinement effects and surface properties has not been fully understood, and the effects of preparation conditions on the physical properties of Group IV nanoparticles remain an open issue. Our simulations are aimed at understanding the physical and chemical properties of C, Si, SiC and Ge nanoparticles with diameters up to 2-3 nm. In particular, we will present investigations of optical gaps and surface properties, and simulations of the effect of different preparation conditions on the structure of Si nanoparticles. Recent simulation results on the early stages of quantum dot functionalization and on salvation properties will also be discussed.
Spectroscopists have become very good at studying fast chemical and materials processes that can be triggered by a pulse of light. However there are a wide variety of processes that cannot be triggered with light. In this talk I will discuss applications of tiny laser-driven shock waves. These are termed "nanoshocks" since one shock wave compresses a volume of about a nanogram and has a duration of a few nanoseconds. Nanoshocks can be used to study fast molecular and material deformation processes. I will discuss how nanoshock waves are generated and detected and how they propagate through condensed matter. Then I will discuss how we use nanoshocks to study void collapse and fracture dynamics in materials, protein dynamics, deformation of individual molecules and nanotechnology high explosives.
The molecular dynamics method is an extremely powerful tool for studying problems in chemistry, biology, physics, and materials science. Starting only with an interatomic potential describing the forces between atoms, a system is propagated forward in time using Newton's laws, and the true dynamical behavior of the system emerges. This approach is now commonly used to study processes such as fast fracture of a solid, structural fluctuations in a protein, liquid diffusion, fast chemical reactions in solution, and sputtering of a solid surface. A major limitation, however, is that because the equations of motion must be integrated with time steps on the order of femtoseconds, the longest time a simulation can run on todays computers is typically less than one microsecond. This precludes direct simulation of a vast number of interesting and technologically relevant processes, such as surface diffusion and surface growth, deformation of a surface during manipulation with an atomic probe, thermally activated annealing processes after a radiation damage event or ion implantation, protein folding, grain boundary diffusion, etc. For many systems, the long-time dynamical evolution consists of infrequent events; the system is caught in a single energy basin, wandering around vibrationally millions or billions of times before finding an escape path (e.g., the hop of a surface adatom) that takes it over a barrier to a new energy basin, where it begins vibrating again. This kind of behavior can be exploited to design methods that reach much longer time scales than direct molecular dynamics. In this talk, I will give an introduction to this new "accelerated molecular dynamics" approach. The key is to let the trajectory find an appropriate way to escape from each basin, but to trick it into doing so more quickly, perhaps at the expense of losing information about the vibrational motion. The result is that the system passes from state to state in a dynamically correct way reaching times of milliseconds, and sometimes even seconds and beyond. Interestingly, the transitions the system makes are often much different than we would have expected from our intuition. I will explain how these methods work, and present results from our simulations on metallic surface growth, deformation and dynamics of carbon nanotubes, and annealing after radiation damage events in MgO.
- Department of Physics and Astronomy
- University of Southern California
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- ACB 439
- Los Angeles, CA 90089-0484
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